One method to adjust engine valve timing is described in U.S. Pat. No. 6,321,731. This patent describes simultaneously adjusting retarded intake and exhaust valves during engine operation to alter engine breathing characteristics. Intake valve closing timing delay makes it necessary to increase the intake manifold pressure to achieve a desired load. As a result, engine pumping work and fuel consumption are reduced. In addition, engine expansion work is increased by late exhaust valve opening timing and engine emissions are reduced by late exhaust valve closing timing.
While it may be beneficial to operate an engine with retarded intake and exhaust valve timing, it can also lead to a rougher running engine during some conditions. For example, if an engine is operated at idle where engine speed is relatively low, exhaust gas residuals can reduce the burn rate and combustion stability can degrade. The slower burn rate may be attributed to intake and exhaust valve overlap along with the “internal EGR” increase that results from late exhaust valve closing. Consequently, engine operation at idle may be degraded if intake and exhaust valve timing is retarded. On the other hand, the further the intake and exhaust valve timing is advanced, the less emissions and fuel consumption benefit may be realized.